MAX17085B_12 Datasheet, PDF(36/38 Page) Maxim Integrated Products – Integrated Charger, Dual Main Step-Down Controllers, and Dual LDO Regulators

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Integrated Charger, Dual Main Step-Down

Controllers, and Dual LDO Regulators

U Minimize the main SMPS current-sensing errors by

connecting LX3 and LX5 directly to the drain of the

low-side MOSFET. Minimize the charger current-

sense resistor trace lengths, and ensure accurate

current sensing with Kelvin connections.

U When trade-offs in trace lengths must be made, it is

preferable to allow the inductor charging path to be

made longer than the discharge path. For example,

it is better to allow some extra distance between the

input capacitors and the high-side MOSFET than to

allow distance between the inductor and the low-side

MOSFET or between the inductor and the output filter

capacitor.

U Route high-speed switching nodes (BST_, LX_, DH_,

and DL_) away from sensitive analog areas (REF,

VCC, and OUT).

U Refer to the MAX17085B evaluation kit for the layout

example.

Layout Procedure

1) Place the power components first, with ground termi-

nals adjacent (NL_ source, CIN, and COUT). If pos-

sible, make all these connections on the top layer with

wide, copper-filled areas.

2) Mount the controller IC adjacent to the low-side

MOSFET, preferably on the backside opposite NL_

and NH_ to keep LX_, GND, DH_, and the DL_ gate-

drive lines short and wide. The DL_ and DH_ gate

traces must be short and wide (50 mils to 100 mils

wide if the MOSFET is 1in from the controller IC) to

keep the driver impedance low and for proper adap-

tive dead-time sensing.

3) Group the gate-drive components (BST_ capacitor,

LDO5 bypass capacitor) together near the controller IC.

4) Make the DC-DC controller ground connections as

shown in Figure 1. This diagram can be viewed as

having two separate ground planes: power ground,

where all the high-power components go, and an ana-

log ground plane for sensitive analog components.

The analog ground plane and power ground plane

must meet only at a single point directly at the IC.

5) Connect the output power planes directly to the out-

put filter capacitor positive and negative terminals

with multiple vias. Place the entire DC-DC converter

circuit as close to the load as is practical.

6) Use a single-point star ground placed directly below

the part at the PGND pin. Connect the power ground

(ground plane) and the quiet ground island at this

location.

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